Gobies and blennies are two groups of small bottom-dwelling fish species that live in identical habitats and look very much alike, even though they belong to different families. Both families are very large – the Gobiidae family includes more than 2,000 species, while the suborder of Blennioidei comprises more than 830 species grouped under around 130 genera.
Both gobies and blennies are benthic animals, which means that they live on or very close to the bottom of the sea. They can be found in the shallow waters all over the world, including not only marine habitats near the shores, but also lakes and brackish waters, where fresh water and salt water mix. They can be found on and around coral reefs as well.
Both gobies and blennies are small fish, with elongate bodies. They are both groups that display considerable diversity in terms of size: some goby and blenny species are only 1 – 1.6 cm when fully grown, while others can reach 30 cm. The two groups look very similar, but they also present some differences. The body of blennies is covered in a kind of mucus, while gobies have tiny scales; blennies have one continuous dorsal fin, while gobies have two separate dorsal fins; blennies have comb-like teeth, while gobies have very small teeth. Given the conditions in their preferred habitat, neither gobies, nor blennies are very good swimmers and they both lack swim bladders (the gas-filled organs that most fish have and has the function of controlling buoyancy), but they choose different methods to propel themselves: blennies have undulating movements, while gobies dart around in the substrate.
Both groups of species dwell the bottom of the sea, but they prefer different menus. Blennies usually fed on crabs and barnacles, while gobies eat small warms and crustaceans.
Like most other bottom dwelling fish species, blennies and gobies are characterized by sexual dimorphism, meaning that males look different from females, usually being larger. Some goby and blenny species are known to be able to change sex during their lifetime – some animals start their life as females, but later on they become transformed into males. Both gobies and blennies lay their eggs on the bottom of the water, usually several thousand of them at a time. The eggs are fertilized externally by the male, after which the male guards them driving predators away and fanning the eggs to provide them with oxygen while the female takes care of the burrow. The eggs take only a few days to hatch into transparent larvae that reach adulthood depending on how favorable the circumstances are. In warmer waters, the young develop faster, reaching adulthood in a few months, but in colder water the process of development may last up to 2 years. The lifespan of gobies varies between 1 and 10 years, while there is still no reliable information about how long blennies live exactly.